1. Field of the Invention
This invention relates to electronic device packaging and, in particular, to the formation of a package including a heat slug. More particularly, this invention relates to the formation of a package with a heat slug via a "drop-in" technique such that a surface of the heat slug is exposed to the exterior of the package.
2. Related Art
In early semiconductor packaging, the integrated circuit was typically packaged either in a metal can or between a ceramic lid and base. Metallic and ceramic packaging materials both provided excellent thermal properties, but each also necessitated expensive and time consuming packaging techniques and materials. For example, the two ceramic substrates used in ceramic packages comprised a sizeable percentage of the total cost of manufacturing the component.
As semiconductor production volumes grew, the need for more cost effective packages grew also. Several new packaging approaches were developed. The most notable was the plastic molded package. However, though the plastic package provided significant cost savings, the advantageous thermal properties present with the use of metallic or ceramic materials were lacking. As integrated circuit speed and density increased, the need for improved thermal performance (i.e., improved dissipation of heat) became more important. This need motivated the inclusion of a metallic heat sink within the package to remove heat from the semiconductor die area.
several approaches that attempt to improve heat dissipation in this way have been tried, the most common being incorporation of a heat spreader into the package. Typically, heat spreaders have been made of aluminum. FIGS. 1A and 1B show a plan view and side view, respectively, of a prior art heat spreader 100. Die attach pad attachment surface 101 of the heat spreader 100 is raised above base surface 100a. Legs 102 extend from base surface 100b that is opposite base surface 100a. Through holes 103 are formed at locations around the periphery of the heat spreader 100.
FIG. 2 shows a plan view of a semiconductor die 220 mounted on a leadframe 221. The leadframe 221 is comprised of a frame 201, leads 221a, tie bars 202 and die attach pad 222. Holes 241 are formed at spaced apart locations in the frame 201. The die attach pad 222 is formed in the center of the leadframe 221. The die 220 is attached to a surface 222b of the die attach pad 222. Tie bars 202 connect the die attach pad 222 to the frame 201. Tie bar downsets 205 facilitate flexure of the tie bars 202. The leads 221a are formed circumferentially around the die attach pad 222. The leads 221a extend from the frame 201 toward the die attach pad 222, but they do not touch it. This is so that, in the finished package the leads 221a may be electrically isolated from the die attach pad 222. This electrical isolation is accomplished, after the integrated circuit is encapsulated, by cutting off the frame 201, leaving extended and unconnected leads 221a and tie bars 202. Bond wires 223 connect the interior end of each of the leads 221a to selected die contact pads 203, providing electrical connection from the integrated circuit on the die 220 to electrical components outside the package.
FIG. 3 shows a cross-sectional view of the heat spreader 100, leadframe 221, and die 220 disposed in the mold cavity 310 of a mold 300. In one method of encapsulating an integrated circuit with heat spreader, the heat spreader 100 is "dropped in" the mold cavity 310 so that legs 102 contact surface 311. The die 220 is attached to the die attach pad attachment surface 222b of the leadframe 221. Bond wires 223 are used to electrically connect the leads 221a to selected bond pads (not shown) on the die 220. The leadframe 221, die 220 and bond wires 223 are then placed in the mold cavity 310, as shown in FIG. 3, so that surface 322 of the die attach pad 222 contacts the die attach pad attachment surface 101 of the heat spreader 100. The leadframe 221 is mounted to the mold half 301 by placing the holes 241 in the frame 201 over the pins 340.
The two mold halves 300a, 300b are closed together. The combination of the thickness 330a of the heat spreader 100 and the dimension 330b is greater than the corresponding height 331 of the mold cavity 310. Therefore, when the two mold halves 300a, 300b are fully closed together, the die attach pad 222 will be pushed upward, causing the tie bars 202 to be bent upward. This upward pull on the tie bars 202 gives rise to tensile forces which act to pull down the end of the tie bars 202 adjacent the die attach pad 222. Consequently, the heat spreader 100 is forced against the mold cavity surface 311 so that it is held in place.
After the mold 300 is closed, encapsulant is transferred into the mold cavity 310 through the channel 335 until the cavity 310 is full. When the encapsulant solidifies, the mold 300 is opened and the completed package removed.
Packages with heat spreaders as described above do not provide the level of heat dissipation required by newer generations of integrated circuit assemblies. Heat spreaders work by transferring heat to the outer regions of the package after the heat is transmitted from the die to the heat spreader. From these outer regions, the heat is then transferred through the encapsulant to the exterior of the package. Since the heat spreader material (e.g., aluminum) is more conductive than plastic encapsulant, heat is dissipated more quickly from the package interior than it would be if the spreader were not present. This turns out, however, not to be fast enough.
Other approaches to the problem of heat dissipation from integrated circuit packages have attempted to expose the surface of the heat sink opposite the die attach pad attachment surface directly to the exterior of the package, thereby greatly reducing the thermal resistance attributable to the presence of the plastic encapsulant. These attempts have been unsuccessful when used with the "drop in" technique due to various manufacturing difficulties. The high pressures present within the mold cavity during the encapsulation process, characteristics of the encapsulant material (e.g., viscosity), and dimensional variations from piece to piece between particular heat sinks have combined to produce separation between the surfaces of the heat sink and mold cavity during encapsulation. This inadequate sealing allows encapsulant bleed (the undesirable presence of translucent encapsulant) or flash (the undesirable presence of encapsulant greater in thickness than bleed and visible to the naked eye) formation on the heat sink surface that was intended to be exposed. Bleed and flash are undesirable both because they degrade the heat transfer capability of the heat sink and because they are undesirable cosmetic defects that most customers will not accept in the finished product. This unwanted plastic necessitates extensive and expensive cleaning of the heat sink surface prior to subsequent processing operations.